The enzyme dipeptidyl peptidase IV (EC.3.4.14.5, abbreviated in the following as DPP-IV) is involved in the regulation of the activities of several hormones. In particular DPP-IV is degrading efficiently and rapidly glucagon like peptide 1 (GLP-1), which is one of the most potent stimulator of insulin production and secretion. Inhibiting DPP-IV would potentiate the effect of endogenous GLP-1, and lead to higher plasma insulin concentrations. In patients suffering from impaired glucose tolerance and type 2 diabetes mellitus, higher plasma insulin concentration would moderate the dangerous hyperglycaemia and accordingly reduce the risk of tissue damage. Consequently, DPP-IV inhibitors have been suggested as drug candidates for the treatment of impaired glucose tolerance and type 2 diabetes mellitus (e.g. Villhauer, WO98/19998). Other related state of the art can be found in WO 99/38501, DE 19616486, DE 19834591, WO 01/40180, WO 01/55105, U.S. Pat. No. 6,110,949, WO 00/34241 and U.S. Pat. No. 6,011,155.
Furthermore, DPP IV contributes to the generation and modulation of a T cell immune response. DPP IV (also known as CD26) has an essential role in immune regulation as a T cell activation molecule and a regulator of chemokine function thus suggesting a role for DPP-IV in the pathophysiology of immune-mediated disorders as well as autoimmune diseases (Hosano O. et al., Modern Rheumatology 2003, 13(3), 199-204). Abnormal expression of DPP-IV is found in the case of autoimmune diseases, HIV-related diseases and cancer. Natural substrates for DPP-IV are involved in immunomodulation, psycho/neuronal modulation and physiological processes in general (Boonacker E.; Van Noorden C. J. F, European Journal of Cell Biology 2003, 82(2), 53-73). Furthermore, it has been shown that there is a correlation between DPP-IV and the key nuclear protein topoisomerase alpha (Aytac U., Dang, N. H., Current Drug Targets: Immune, Endocrine and Metabolic Disorders 2004, 4(1), 11-18). Thus, DPP-IV inhibitors may be useful as medicaments for the treatment of various diseases in which DPP-IV is involved.